(1) Field of the Invention
The present invention relates to a scroll type refrigerant compressor which is adapted to use in the refrigerating system of an automobile.
(2) Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 61-76782 discloses a scroll type refrigerant compressor adapted for use in the refrigerating system of an automobile. The compressor comprises front and rear housings connected to each other, a drive shaft held in the housings for rotation, and an electromagnetic clutch provided on the drive shaft to transmit the power from the automobile engine. The front housing has a suction chamber. The rear housing has a discharge chamber and a low pressure chamber provided around the discharge chamber.
The rear housing accommodates a stationary scroll unit fixedly mounted thereto and a movable scroll unit which engages the stationary scroll unit. The stationary scroll unit comprises a stationary end plate and a spiral member attached to the stationary end plate. The movable scroll unit comprises a movable end plate disposed axially opposite to the stationary end plate, and a movable spiral member attached to the movable end plate. The movable scroll unit is connected to the drive shaft so as to move along an orbiting path.
The stationary and movable scroll units define compression chambers therebetween. The compression chambers shift from the periphery to the central portion of the scroll units due to the orbiting motion of the movable scroll unit. The volume of the moving compression chambers are progressively reduced. Refrigerant gas introduced from the suction chamber into the compression chambers is compressed by the shifting compression chambers to the central portion of the scroll units.
The stationary scroll unit includes relief ports formed in the stationary end plate to communicate the compression chambers with the low pressure chamber. The compressor further comprises a valve plate in the form of a ring member provided on the outer face of the stationary end plate for rotation between a first position and a second position. The valve plate includes passages which are aligned with the relief ports to allow the compression chamber to communicate with the low pressure chamber when the valve plate is at the first position, and to stop the compression chambers communicating with the low pressure chamber when the valve plate is at the second position.
The compressor further comprises an actuator for rotating the valve member in the low pressure chamber. The actuator is provided with a cylinder having a distal end chamber and a proximal end chamber, a piston slidably provided in the cylinder and outwardly extending from the distal end of the cylinder, and a spring in the proximal end chamber for biasing the piston toward the distal end of the cylinder. The outer end of the piston is connected to the valve plate to rotate it to the first position when the piston is advanced from the cylinder, and to the second position when the piston is retracted into the cylinder. The distal end chamber is fluidly connected to the compression chambers via a conduit, and the proximal end chamber is fluidly communicated with the low pressure chamber through a passage in the cylinder wall.
When the compressor starts its operation, the pressure in the compression chamber has not increased, and the piston is advanced from the cylinder by the biasing force of the spring. Thus, the valve plate rotates to the first position to allow the compression chamber to communicate with the low pressure chamber, which reduces the displacement of the compressor and the shock generated at the start of the compressor.
After the start of the compressor, the piston is displaced by the pressure in the distal end chamber of the cylinder when the pressure in the compression chamber reaches a certain pressure level, which is transmitted to the distal end chamber via the conduit. Thus, the valve plate is rotated to the second position to separate the compression chamber from the low pressure chamber, which increases the displacement of the compressor to the maximum.
In the prior art compressor described above, the on-off operation of the compressor is controlled by engaging and disengaging an electromagnetic clutch provided on the drive shaft. Therefore, the prior art compressor must have an electromagnetic clutch, which increase the weight and fuel consumption of the automobile on which the compressor is mounted.
Further, in the prior art compressor described above, there is a problem of frosting on the refrigerating circuit during high speed operation of the compressor, since the compressor cannot function at a reduced displacement.
The invention is directed to a solution of the problem of the prior art.